🧪 Add tests for get_export_cases API

[manupawickramasinghe]

🎯 What: Added missing test coverage for get_export_cases (GET /cases) API endpoint in frigate/test/http_api/test_http_export.py.

📊 Coverage: Covered scenarios:

• GET /cases returning an empty list when no cases exist.
• GET /cases returning a list of cases correctly ordered by created_at descending.

✨ Result: Improved test coverage and reliability for export case listing API.

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PR created automatically by Jules for task 6706424678628255419 started by @manupawickramasinghe

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